Many sound absorbing mufflers for flowing gas systems have utilized a variety of chronologically developed generic design techniques. One technique involves "scooping" some of the gas flowing through the muffler to excite resonant chambers found within the muffler. Unfortunately, scooping the gas requires interrupting the flow of the gas which can create significant back pressure in the system. Accordingly, if the scooped gas technique were used in the design of an automobile muffler, a significant loss in engine efficiency would result.
In order to alleviate the problems caused by interrupting the gas flow, another technique involves placing the resonant chambers within the expansion chamber. However, these types of mufflers are limited to attenuating specific tones or frequencies. Thus, broadband sound and high frequency noise is largely unaffected and may radiate from the muffler.
Accordingly, the next phase of muffler development addressed the aforementioned limited frequency drawbacks. To do so, acoustically absorptive materials were employed to dissipate broad frequencies of sound within muffler designs that permitted a largely unrestricted gas flow through the muffler. Unfortunately, the size and weight of these designs proved prohibitive in many space sensitive applications.